Thermal stable synthetic zeolite

ABSTRACT

An ultra thermal stable zeolite aluminosilicate having a silica to alumina ratio of 2.5 to &lt;3.5 and a process of making this composition characterized by ammonia ion exchange, followed by calcination at from 1000° to 1400°F. The composition is further characterized by sodium content expressed as the oxide of less than 4 percent.

This application is a continuation-in-part of our earlier application Ser. No. 233,827, filed Mar. 10, 1972, now abandoned, which in turn is a continuation of application Ser. No. 29,285, filed Apr. 16, 1970, now abandoned, which is a continuation-in-part application of earlier application Ser. No. 714,357, filed Mar. 19, 1968, now U.S. Pat. No. 3,518,051, which is a continuation-in-part application of our earlier application, Ser. No. 367,864, filed May 15, 1964, now abandoned.

This invention relates to new microselective zeolites that have high steam and thermal stability.

The microselective adsorbents known as molecular sieves are zeolites, that is, crystalline metal aluminosilicates, with three-dimensional structures of silica-alumina tetrahedra. The zeolite structure is characterized by a repeating three-dimensional network of large open aluminosilicate cages interconnected with smaller uniform openings or pores. Certain of these microselective adsorbents have been prepared synthetically from sodium silicate and sodium aluminate. After synthesis, these large cavities are filled with water which can be driven off by heating without collapsing the case. When dehydrated, these cavities can readsorb large quantities of water and other vapors at low partial pressures. Due to the uniform structures or pore openings connecting the aluminosilicate cavities, these zeolites exhibit the unique property of excluding large molecules to pass through and be adsorbed, thereby acting as microselective adsorbents for molecules according to their size and shape.

It has been shown that adsorbents having an effective pore size of about 4A units are of particular interest in adsorbing vapors of low molecular weight materials, such as ethane, ethylene and propylene from mixtures of the same. Other adsorbents having much larger effective pore sizes are also of interest in adsorbing large molecules and storing them for later release. These microselective adsorbents will adsorb molecules whose critical size is less than about 10 to 14A units. These materials can function as carriers for a wide variety of chemical compounds. When the chemical compound is needed, it is released by heating the carrier or by displacement of the chemical with another adsorbent such as water. In addition, these zeolites also exhibit the property of base exchange, wherein one metal cation can be exchanged for another, allowing controlled changes in effective pore size.

There has been considerable interest in the use of these molecular sieve material in the production of catalysts. Certain of these materials have been used in the production of hydrocracking catalyst. So-called "metal loaded" molecular sieves, that is, molecular sieves wherein a portion of the ions have been ion exchanged with certain metals, have been suggested for use in other types of catalytic reactions in organic chemistry.

In copending application Ser. No. 242,892, filed Dec. 7, 1962 and now abandoned, the use of these silica-alumina composites containing zeolitic materials as fluid cracking catalysts is discussed. These catalysts have very desirable cracking properties. Their cracking activity is excellent. The gas producing factors and carbon producing factors are within desirable ranges and the products recovered are those most desirable in refinery operation.

One of the principal problems encountered in incorporating these microselective adsorbents into the silica-alumina fluid cracking catalyst is their general lack of structural stability. This is very important because the regeneration cycle of a fluid cracking catalyst requires the catalyst be able to withstand steam and/or thermal atmospheres in the range of 1300°-1700°F. When fluid cracking catalysts are regenerated, the temperature of the catalyst frequently reaches a high level. Any catalytic system that cannot withstand such temperatures loses its catalytic activity on regeneration and its usefulness is greatly impaired.

In the preparation of silica-alumina fluid cracking catalysts by conventional techniques, elaborate methods are prescribed and followed to remove the sodium from the cracking catalyst. In the preparation of synthetic 13 percent "active" alumina and 25 percent "active" alumina cracking catalysts, the level of sodium in the final product is reduced to a minimum. This is necessary because the presence of sodium in these catalysts is one of the principal causes of the lack of structural stability at high temperatures.

Most zeolites are prepared or occur naturally in the alkali metal or mixed alkali meal-alkaline earth form. In order to obtain a stable catalyst, it is important that the zeolite be structurally stable. In the application of these materials in cracking catalysts, the zeolite will comprise about 2 to 90 percent of the resulting composite with the preferred range being about 5 to 25 percent.

The stabilization of zeolites with a silica to alumina ratio of approximately 3.5-7, is described in our U.S. Pat. No. 3,293,192. It has been believed for some time that it is impossible to stabilize those zeolites having a relatively low silica to alumina ratio of 2.5 to less than 3.5. U.S. Pat. No. 3,130,006 discusses the problems believed to be inherent in any attempts to stabilize low silica-containing zeolites. In this patent the patentee states that when the zeolite with a silica to alumina ratio of 2.5± 0.5 is converted to the ammonium form and subsequently heated, the zeolite collapses to an amorphous material and loses its internal pore structure and practically all of its x-ray identification.

We have found that contrary to the widely held theories expounded in the prior art patents, as well as in the literature, a unique zeolite composition in the hydrogen form which is stable to high temperature, thermal and steam treatment can be prepared in which the silica to alumina ratio is 2.5 to less than 3.5. The zeolite is stable to thermal treatment at temperatures as high as 1650°F. Another distinguishing feature of the unique zeolite composition of our invention is the unit cell size. The unit cell of this composition is smaller than the unit cell of the parent zeolite. This decrease in unit cell size is achieved by the unique stabilization process. The basic formula for our crystalline zeolite may be represented as follows:

    XM.sub.2/n O.Al.sub.2 O.sub.3. 2.5-<3.5 SiO.sub.2.YH.sub.2 O

where M represents any cation and n is its valence, where Y can vary from 0 to 9, and X from 0 to 0.5. The stabilized zeolite can contain varying amounts of cations or be cation-free. One particularly significant value of our product is the extra stability attained by elimination of the alkali metal ion and the resulting unit cell shrinkage during the stabilization step. In addition to its structural stability at high temperature, the unique product is identified both by its smaller unit cell and the reduction of alkali metal in the crystal structure.

The chemical analysis range of our cation free form of these stabilized zeolites hereinafter referred to as Z-14XS is as follows:

                  Table 1                                                          ______________________________________                                         SiO.sub.2 (dry basis)                                                                              52 to 64 25. %                                             Al.sub.2 O.sub.3 (dry basis)                                                                       33 to 46 wt. %                                             Na.sub. 2 O (dry basis)                                                                            0.1 to 1.0 wt. %                                           ______________________________________                                    

Our extra stable zeolite can be prepared by various processes. In one process, the zeolite is stabilized by incremental exchange of the alkali metal ions.

We have found that during one method of preparation of our extra stable zeolite product, the zeolite goes through stages in which it is in the NH₄ + form and the H⁺ form and that it is finally stabilized as Z-14XS, as shown by the unit cell shrinkage and high stability. Under certain conditions, other cations may then be introduced to produce a highly stable cation form of Z-14XS. Suitable cations include complex amine ions, yttrium, the rare earths, aluminum, platinum, other group VIII, IB, IIB, IIA, IIIB, IVB, VB, VIB, VIIB, metal ions, etc., but not the alkali metals.

Although many uses will be suggested to those skilled in the art for our novel zeolite, these products have a particularly useful application as components of catalysts because of their stability, activity and selectivity. Certain of these zeolites do not lose their crystal structure either during thermal calcination at temperatures as high as 1700°F. or steam treatment with 25 percent steam at a temperature of 1525°F. for 16 hours. This Z-14XS product, like the Z-14US product, described in our U.S. Pat. No. 3,293,192, is particularly attractive as a component of fluid cracking catalyst. Other catalytic uses would be important.

Broadly speaking, one of the processes for making our novel product consists of selecting a zeolitic material having a silica to alumina ratio of 2.5 to less than 3.5 and base exchanging 65 percent or less (preferably 10 to 65 percent) of the alkali metal ion with an ammonium salt, amine salt or other salt which decomposes on calcination to leave appreciable portions of the zeolite in the hydrogen form. This material is then calcined at temperatures in the order of about 1000° to 1400°F., preferably about 1250° to 1350°F., depending on the Na₂ O content at the time of calcination. After this treatment, the exchange is repeated so that the sodium is removed in increments of 10 to 80 percent based on the sodium remaining in the zeolite. Each successive exchange is followed by a calcination as set out above. This exchange and calcination treatment is repeated successively until the Na₂ O content is in the proper range, that is, less than about 1 percent by weight, preferably 0.1 to 1.0. After the final exchange, the material is then filtered, washed and given a final stabilization by calcination.

The first step in our process is the selection of a suitable zeolite. The initial zeolites used in the process of the instant application are typically synthetic faujasite zeolites having a silica to alumina ratio of 2.5 to less than 3.5. Examples of suitable starting zeolites are described in U.S. Pat. No. 2,882,244 and U.S. Pat. No. 2,979,381. The latter patent describes a suitable method of preparing this zeolitic material.

The next step of one of our processes is the base exchange of a portion of the sodium in the zeolite with an ammonium salt, amine salt or other salt, which on calcination decomposes to leave appreciable portions of the zeolite in the hydrogen or cation free form. Examples of suitable compounds of this type include ammonium chloride, ammonium sulfate, tetraethyl ammonium chloride, tetramethyl ammonium sulfate, etc. Ammonium salts, because of their ready availability and low costs, are the preferred reagents in this step of the reaction. This exchange is carried out rapidly with a small amount of salt solution. The salt should be present in sufficient quantity to exchange 10 to 65 percent of the sodium cations initially present in the zeolite. The exchange is carried out rapidly at 100°C. However, temperatures in the range of 25° to 125°C., preferably 50° to 100°C., give satisfactory results. This exchange is carried out until approximately 10 to 65 percent of the sodium has been exchanged with ammonium. The exchange is complete in a period of about 5 minutes to 24 hours. The longer times have no adverse effect, but no advantage is gained by their use.

After the preliminary exchange is completed, the zeolite is filtered, washed and dried. The composite is then calcined or steamed mildly at temperatures in the range of about 1000° to 1400°F. The calcination is preferably carried out at temperatures of about 1250° to 1350°F for a period of 0.15 to 12 hours at the calcination temperature.

In the remaining steps of the process this exchange and calcination is repeated until the Na₂ O content of the zeolite is reduced to about 0.1 to 1 percent by weight. A somewhat larger portion of the remaining Na₂ O can be removed with each succeeding step as expressed by the relation

    X ≧ (.4).sup.1.2 g

wherein X is the fraction of the original Na₂ O remaining, and g is the number of exchanges. To obtain the desired final product X ≦ 0.064 and g ≧ 3.

By the term "alkali metal," we intend to include the elements in Group IA, lithium through cesium. By the term "rare earth elements," we intend to include the elements lanthanum to lutecium, atomic numbers 57 to 71.

In addition to its stability, the product of our invention is characterized by the unit cell size, the water adsorption or desiccant properties and the ion exchange capacity.

The unit cell of our novel product shrinks from the normal 24.80 to 25.00A, which is characteristic of the zeolitic starting material, to 24.45 to 24.70A. This unit cell shrinkage occurs in the repeated stabilization steps.

Unit cell measurements were made on our Z-14XS materials, and the unit cell dimensions were noted. This was done using a Norelco x-ray diffractometer with a nickel-filtered copper K radiation. The instrument was operated with 40KV operating potential and 20 ma. The sample to be run was mixed with about 10 percent of a suitable inert internal standard and scanned from about 45° two-theta to about 60° two-theta at a goniometer speed of 1/4° per minute and a chart speed of 1/2 inch per minute. The settings for the electronic circuit panel were as follows for the first diffraction line (sodium chloride):

           scale factor     8                                                             scale multiplier 1                                                             time constant    4                                                 

The remainder of the scan was obtained at

           scale factor     4                                                             scale multiplier 1                                                             time constant    8                                                 

At no time during the scan was the goniometer drive or chart drive stopped.

The observed and theoretical (from National Bureau of Standards Circulars) values for the internal standard were used to correct systematic errors in observed values of two-theta.

Our Z-14XS product may be characterized as having a unit cell of 24.45 to 24.70 and having a silica to alumina ratio of 2.5 to less than 3.5 and by its thermal stability to high temperature treatment.

The thermal stability of the Z-14XS is demonstrated by the surface area of the product after calcination at temperatures up to 1650°F. for 16 hours and steam treatment with an atmosphere of 25 percent steam at a temperature of 1525°F. for 16 hours. Our novel product retains a surface area of greater than 100 m² /g after this thermal treatment and greater than 200 m² /g after the steam treatment described above.

These criteria clearly distinguish this product from the starting non-stabilized zeolite and from other reported non-stabilized zeolites of similar chemical composition.

The invention is further illustrated by the following specific but non-limiting examples. In all cases, unless otherwise noted, all parts and percentages are by weight.

EXAMPLE 1

A synthetic faujasite having a silica to alumina ratio of 2.71 was prepared as follows:

1. 43.6 g. of Na₂ O:Al₂ O₃.3 H₂ O was dissolved in 186 g. of water with 32 g. of sodium hydroxide. The solution was cooled to room temperature.

2. 38.4 g. of fine sized amorphous silica (HiSil) was calcined at 1000°F. for 3 hours and mixed with 228 g. of water.

3. Solution (1) and mixture (2) were mixed together and aged at room temperature for 4 days.

4. The mixture was then heated to 100°C. for 6 hours. The solid zeolite was removed by filtration and washed with water.

The product possessed the following characteristics:

    Surface area       732 m.sup.2 /g                                              Na.sub.2 O         17% by wt.                                                  Al.sub.2 O.sub.3   31.15% by wt.                                               SiO.sub.2          49.59% by wt.                                               SiO.sub.2 /Al.sub. 2 O.sub.3                                                                       2.71                                                       Unit Cell          24.85A                                                 

A 15 g. sample of the above zeolite was ammonium exchanged at 100°C. for 30 minutes in a solution containing 2 g. (NH₄)₂ SO₄ dissolved in 100 g. H₂ O. The zeolite was then recovered by filtration, washed with water, and calcined 2 hours at 1300°F. This procedure was repeated for a total of 6 exchanges with a calcination occurring between exchange. The results are tabulated in the following:

             Na.sub.2 O  Surface Area Unit Cell                                    Exchange (% by wt.)  (m.sup.2 /g) (A)                                          ______________________________________                                         None     17.0        732          24.85                                        1        13.1        672          --                                           2        9.4         660          --                                           3        5.9         468          --                                           4        3.4         385          --                                           5        1.7         314          --                                           6        0.97        284          24.67                                        ______________________________________                                    

As is seen from the above, the product undergoes a unit cell shrinkage of 0.18A. The thermal stability of the final product was determined by calcination at 1600°F. for 2 hours, whereupon it was found that the zeolite retained a surface area of 280 m² /g. A 2-hour calcination at 1650°F. showed a surface area retention of 205 m² /g. A sample of the same zeolite, which was not subjected to the present stabilization procedure, was found to retain essentially no surface area upon heating to 1600°F. for 2 hours.

EXAMPLE 2

A synthetic faujasite having a silica to alumina ratio of 3.03 was prepared using a procedure similar to that set forth in Example 1.

A 100 g. sample of this zeolite was ammonium exchanged with ammonium sulfate solution using a zeolite to ammonium sulfate to water ratio (by weight) of 1:1:10. The exchange was conducted at 100°C. for 30 minutes. The product was recovered by filtration, washed with water and calcined for 2 hours at 1300°F. This procedure was repeated for a total of four exchanges with a calcination occurring between each exchange. The results are tabulated below:

    Na.sub.2 O Surface Area                                                                           Surface Area  Unit Cell                                                        (after 2 hrs)                                               Exchange                                                                            (% by wt.)                                                                           (m.sup.2 /g)                                                                           at 1650°F.                                                                      1700°F.                                                                       (A)                                           __________________________________________________________________________     None 17.0  974     --      --    24.76                                         1    5.98  573     --      --    --                                            2    1.97  514      27     --    --                                            3    0.82  464     369     256   --                                            4    0.29  441     361     298   24.54                                         __________________________________________________________________________

The above data shows that this zeolite underwent a unit cell shrinkage of 0.22A and retains substantial surface area after heating at 1700°F. for 2 hours.

EXAMPLE 3

A second sample of the zeolite of Example 2 was exchanged 4 times as set forth in Example 2 to reduce the Na₂ O level to 0.38 percent by weight. However, no calcination was conducted between the exchanges.

The final product after washing was calcined for 6 hours at about 1000°F. The surface area, which was initially 974 m² /g, was reduced to 10 m² /g even though the Na₂ O content had been reduced to 0.38 percent by weight.

EXAMPLE 4

A synthetic faujasite having a silica to alumina ratio of 3.04 was prepared by the procedure of Example 1 using the following ratio of reactants: 3 Na₂ O : Al₂ O₃ : 4 SiO₂ : 120 H₂ O. The final product possessed the following characteristics:

    SiO.sub.2          50.81 % by wt.                                              Al.sub.2 O.sub.3   28.36 % by wt.                                              Na.sub.2 O         16.90 % by wt.                                              SiO.sub.2 /Al.sub.2 O.sub.3                                                                        3.04                                                       Surface Area       896 m.sup.2 /g                                         

Samples A and B of the above zeolite were exchanged with ammonium sulfate solution using a ratio of zeolite to (NH₄)₂ SO₄ to water of 1:1:10. The samples were subjected to calcination as follows:

           Cal. Temp.  Time     Na.sub.2 O                                                                            Surface Area                                Sample (°F.)                                                                               (hr.)    % wt.  m.sup.2 /g                                  ______________________________________                                         A      1300        1        5.8    603                                         B      1300        2        5.8    557                                         ______________________________________                                    

Samples A and B were then exchanged using a zeolite to (NH₄)₂ SO₄ to water ratio of 1:2:20; subdivided to form samples A, A2, B1 and B2; and calcined as follows:

    Sample Na.sub.2 O                                                                              Cal. Temp.   Time  Surface Area                                ______________________________________                                         A1     1.8      1300         1     452                                         A2     1.8      1300         2     497                                         B1     1.7      1300         1     530                                         B2     1.7      1360         2     557                                         ______________________________________                                    

Samples A2, B1 and B2 were again exchanged using a zeolite to (NH₄)₂ SO₄ to water ratio of 1:2:20; subdivided to samples A2-1, A2-2, etc.; and calcined as follows:

    Sample  Na.sub.2 O                                                                               Temp.     Time   Surface Area                                ______________________________________                                         A2-1    0.38      1300      1      442                                         A2-2    0.38      1600      2      462                                         A2-3    0.38      1650      2      414                                         A2-4    0.38      1700      2      319                                         B1-1    0.36      1300      2      449                                         B1-2    0.36      1600      2      385                                         B1-3    0.36      1650      2      344                                         B1-4    0.36      1700      2      331                                         B2-1    0.32      1300      1      492                                         B2-2    0.32      1600      2      391                                         B2-3    0.32      1650      2      384                                         ______________________________________                                    

The Z-14XS product as initially formed is in an essentially decationized form. The zeolite has less than a 1 percent alkali cation content, that is, sodium ion content, with no other cations present. A zeolite having a silica/alumina ratio of 2.5± 0.5 in a decationized form has been generally considered as not possible to synthesize. However, by the present sequential exchange technique, it is possible to make such a decationized zeolite when the silica/alumina ratio is about 2.5 to 3.0. This is the upper part of the range for what is also known as zeolite X. However, it is also feasible by the present technique to produce zeolites in a decationized form wherein the zeolites have a silica/alumina ratio of 3.0 to less than 3.5. These decationized zeolites will have a unit cell size of 24.45 to 24.70 angstroms. This smaller unit cell size is in part responsible for the greater heat and steam stability of the zeolite. After having been formed in this decationized, decreased cell size structure, this ultra stable zeolite can be exchanged with other cations. Although in a cation form such as calcium, rare earth, ammonium or transition metal form, it will have this reduced, more stable cell size which resulted from the sequential ammonium ion exchange with heat treating steps after each ammonium exchange. Further, in order to produce this cell size structure, it is necessary that in addition to the technique of exchange being sequential followed by heat treatment, that it also be only an ammonium ion exchange. An exchange with ammonium and other cations either combined in an exchange solution or in sequential exchanges will not produce a cell size of 24.45 to 24.70 angstroms for a zeolite having a silica/alumina ratio of 2.5 to less than 3.5. 

What is claimed is:
 1. As a composition of matter a synthetic ultra stable zeolite aluminosilicate having the composition expressed in terms of oxide mole ratios as follows

    X M.sub.2/n O: Al.sub.2 O.sub.3 : 2.5 -<3.5 SiO.sub.2 : Y H.sub.2 O

where M represents alkali metal of less than one percent by weight of said zeolite and hydrogen and n is the valence of M, X may vary from 0 to 1 and Y may be any value up to 9, said ultrastable zeolite aluminosilicate further being characterized by: a cubic cell size of 24.45 to 24.70 angstroms and an ultrastable structure as evidenced by retention of a surface area of (i) greater than 100 m² /g after treatment at 1650°F. for 16 hours and (ii) greater than 200 m² /g after treatment with an atmosphere of 25 percent steam at a temperature of greater than 1525°F. for 16 hours.
 2. The composition of claim 1 containing cations selected from the group consisting of complex amine cations, yttrium ion, rare earth ions, Group IB, IIA, IIB, IIIA, IIIB, IVB, VB, VIIB or VIII metal ions, said cations introduced into said ultrastable zeolite aluminosilicate by ion exchange. 